This invention relates to a circuit arrangement for processing sampled analog electrical signals.
U.S. Pat. Nos. 4,864,217 (9/5/89) disclose method of processing sampled analog electrical signals in which the electrical quantity manipulated is current. This method is referred to hereinafter as switched current signal processing and circuit arrangements using this method are referred to as switched current circuits. It is known, in switched capacitor circuits, to manipulate electrical charges to perform signal processing of sampled analog electrical signals. However, in order to manipulate the charges high quality linear capacitors are required and in MOS integrated circuits these are commonly fabricated using two polysilicon layers. The provision of two polysilicon layers is not a standard part of the CMOS processes usually used for LSI and VLSI digital circuits and it therefore makes the provision of circuits combining analog and digital signal processing on a single integrated circuit more difficult.
In addition the capacitors required for the signal manipulation in switched capacitor circuits occupy a large area which can be half or more of the total chip area. By using switched current circuits the processing and chip area problems may be mitigated. However, it has been found convenient to use current mirror circuits in the implementation of switched current circuits and, at least in the simpler embodiments, these circuits require a unidirectional input current. Consequently if, as is usually the case, there is a requirement to process bidirectional input signals it is necessary to add a bias current to the bidirectional input current to ensure that a unidirectional input current is available. However, it is then necessary to ensure that the bias current is not itself processed in the same manner as the bidirectional input current since it will then be difficult to separate it from the processed signal current. This requires additional bias current sources at various positions in the processing circuitry having different current magnitudes. Consequently errors can occur due to the difficulty of producing accurately defined bias currents and in particular matching current sources which may be widely separated over the area of an LSI or VLSI chip.